An inverter with a pull-up device and a pull-down device is a core circuit of many digital circuits such as NAND gates, NOR gates, multiplexers, and decoders. Silicon CMOS technology combines an NMOS device with a PMOS device as an efficient pull-up device.
Compound semiconductor technology lacks a complementary PMOS device. Consequently, with compound semiconductors, the pull-up device is implemented with a resistor or a depletion mode device. In examples where the pull-up device is implemented with a depletion mode device, a current source is coupled to the gate of the depletion mode device and to the source of the enhancement mode pull-down device.